1. Field of the Invention
This invention relates to cryosurgery and more particularly to cryoablation catheters comprising a fluid operating near its critical point.
2. Description of the Related Art
Cryosurgery is a promising approach for treating various medical conditions, none of which are less important than the treatment of an abnormal heart beat.
Atrial flutter and atrial fibrillation are heart conditions in which the left or right atrium of the heart beat improperly. Atrial flutter is a condition when the atria beat very quickly, but still evenly. Atrial fibrillation is a condition when the atria beat very quickly, but unevenly.
These conditions are often caused by aberrant electrical behavior of some portion of the atrial wall. Certain parts of the atria, or nearby structures such as the pulmonary veins, can misfire in their production or conduction of the electrical signals that control contraction of the heart, creating abnormal electrical signals that prompt the atria to contract between normal contractions caused by the normal cascade of electrical impulses. This can be caused by spots of ischemic tissue, referred to as ectopic foci, or by electrically active fibers in the pulmonary veins, for example.
Currently, the Cox Maze procedure, developed by Dr. James Cox in the 1980's, is a method for eliminating atrial fibrillation. In the Cox Maze procedure, the atrial wall is cut with a scalpel in particular patterns which isolate the foci of arrhythmia from the rest of the atrial wall, and then sewn back together. Upon healing, the resultant scar tissue serves to interrupt ectopic re-entry pathways and other aberrant electrical conduction and prevent arrhythmia and fibrillation. There are several variations of the Cox maze procedure, each involving variations in the number and placement of lesions created.
The original Cox maze procedure was an open chest procedure requiring surgically opening the atrium after opening the chest. The procedure itself has a high success rate, though due to the open chest/open heart nature of the procedure, and the requirement to stop the heart and establish a coronary bypass, it is reserved for severe cases of atrial fibrillation.
The Cox maze procedure has been performed using ablation catheters in both transthoracic epicardial approaches and transvascular endocardial approaches. In transthoracic epicardial approaches, catheters or small probes are used to create linear lesions in the heart wall along lines corresponding to the maze of the Cox maze procedure. In the transvascular endocardial approaches, a catheter is navigated through the vasculature of the patient to the atrium, pressed against the inner wall of the atrium, and energized to create lesions corresponding to the maze of the Cox maze procedure.
In either approach, various ablation catheters have been proposed for creation of the lesion, including flexible cryoprobes or cryocatheters, bipolar RF catheters, monopolar RF catheters (using ground patches on the patient's skin), microwave catheters, laser catheters, and ultrasound catheters. U.S. Pat. No. 6,190,382 to Ormsby and U.S. Pat. No. 6,941,953 to Feld, for example, describe RF ablation catheters for ablating heart tissue. These approaches are attractive because they are minimally invasive and can be performed on a beating heart. However, these approaches have a low success rate. The low success rate may be due to incomplete lesion formation. A fully transmural lesion is required to ensure that the electrical impulse causing atrial fibrillation are completely isolated from the remainder of the atrium, and this is difficult to achieve with beating heart procedures.
A major challenge to the effective epicardial application of ablative energy sources to cardiac tissue without the use of the heart-lung machine (“off-pump”) is that during normal heart function the atria are filled with blood at 37° C. that is moving through the atria at roughly 5 liters per minute. If cryothermia energy is applied epicardially, this atrial blood flow acts as a “cooling sink,” warming the heart wall and making it difficult to lower the endocardial surface of the atrial wall to a lethal temperature (roughly −30° C.). Thus, lesion transmurality is extremely difficult to attain.
Similarly, if heat-based energy sources such as RF, microwave, laser, or HIFU are applied to the epicardial surface without using the heart-lung machine to empty the atria, the blood flowing through the atrium acts as a heat sink, cooling the heart wall making it difficult to raise the endocardial surface of the atrial wall to a lethal temperature (roughly 55° C.).
Another shortcoming with certain cryosurgical apparatus arises from evaporation. The process of evaporation of a liquefied gas results in enormous expansion as the liquid converts to a gas; the volume expansion is on the order of a factor of 200. In a small-diameter system, this degree of expansion consistently results in a phenomenon known in the art as “vapor lock.” The phenomenon is exemplified by the flow of a cryogen in a thin-diameter tube, such as is commonly provided in a cryoprobe. A relatively massive volume of expanding gas that forms ahead of it impedes the flow of the liquid cryogen.
Traditional techniques that have been used to avoid vapor lock have included restrictions on the diameter of the tube, requiring that it be sufficiently large to accommodate the evaporative effects that lead to vapor lock. Other complex cryoprobe and tubing configurations have been used to “vent” N2 gas as it formed along transport tubing. These designs also contributed to limiting the cost efficacy and probe diameter.
Due to the nature of the procedure and anatomical locations that lesions must be placed, the cryoprobe must be sufficiently flexible by the surgeon to be placed on the correct location of the heart surface.
Malleable and flexible cryoprobes are described in U.S. Pat. Nos. 6,161,543 and 8,177,780, both to Cox et al. The described probe has a malleable shaft. In embodiments, a malleable metal rod is coextruded with a polymer to form the shaft. The malleable rod permits the user to plastically deform the shaft into a desired shape so that a tip can reach the tissue to be ablated.
U.S. Pat. No. 5,108,390, issued to Potocky et al, discloses a highly flexible cryoprobe that can be passed through a blood vessel and into the heart without external guidance other than the blood vessel itself.
Several patents disclose the use of bellows-type assemblies for use with cryoablation systems. For example, U.S. Pat. No. 6,241,722, issued to Dobak et al, discloses a cryogenic catheter with a bellows and which utilizes a longitudinally movable Joule-Thomson nozzle of expansion. The Dobak '722 device preferably uses closed media-flow pathways for recycling of the media employed.
Dobak et al, in U.S. Pat. No. 5,957,963, disclose the use of a flexible catheter inserted through the vascular system of a patient to place the distal tip of the catheter in an artery feeding a selected organ of the patient. The '963 patent discloses a heat transfer bellows for cooling the blood flowing through the artery.
U.S. Pat. No. 6,767,346, issued to Damasco et al, entitled, “Cryosurgical Probe With Bellows Shaft”, discloses use of a cryosurgical probe with a bellows shaft. U.S. Pat. No. 6,936,045, issued to Yu et al, entitled, “Malleable Cryosurgical Probe” discloses a cryosurgical probe used for Joule-Thomson nozzles.
CryoCath Technologies, Inc., Montreal, Quebec, Canada, utilizes a cryoablation probe trademarked under the name Surgifrost® which involves the use of a cryoprobe with a malleable or corrugated shell.
A problem with this and other similar products, however, is that these cryoprobes are not sufficiently flexible during use. Cryogenic temperatures tend to make metals and alloys more rigid, and less flexible. Such cryoprobes and catheters may not be articulated nor have the flexibility to form the necessary and desired shape when a cryogenic fluid is circulated through the treatment section of the apparatus. As a result, there is often an incomplete/intermittent thermal contact along the whole line of freezing. The small contact area provides a limitation for the power delivered to the tissue.
Additionally, there are substantial limits on flexibility and conformability of the treatment regions of the cryoablation apparatus. If the distal treatment section is too delicate and a breach in the cover occurs, cryogen may leak into the bloodstream. Substantial danger may result, perhaps death. Bubbles and/or cryogen in the heart, for example, may be immediately sent to the vessels in the brain. Such circumstances may result in highly undesirable neuro-ischemic events.
Various others have attempted to reduce the likelihood of a cryogenic fluid leaking into the bloodstream. U.S. Pat. No. 7,648,497 to Lane, for example, provides a second balloon surrounding a first balloon. The space between the first balloon and the second balloon is under vacuum. However, use of vacuum is undesirable because it is a very weak thermal conductor. Use of a weak thermal conductor reduces cooling power.
There is accordingly a need for improved methods and systems for providing minimally invasive, safe and efficient cryogenic cooling of tissues.